clang is a C, C++, and Objective-C compiler which encompasses
preprocessing, parsing, optimization, code generation, assembly, and linking.
Depending on which high-level mode setting is passed, Clang will stop before
doing a full link. While Clang is highly integrated, it is important to
understand the stages of compilation, to understand how to invoke it. These
stages are:

Driver

The clang executable is actually a small driver which controls the overall
execution of other tools such as the compiler, assembler and linker.
Typically you do not need to interact with the driver, but you
transparently use it to run the other tools.

Preprocessing

This stage handles tokenization of the input source file, macro expansion,
#include expansion and handling of other preprocessor directives. The
output of this stage is typically called a ".i" (for C),
".ii" (for C++), ".mi" (for Objective-C), or
".mii" (for Objective-C++) file.

Parsing and Semantic Analysis

This stage parses the input file, translating preprocessor tokens into a
parse tree. Once in the form of a parse tree, it applies semantic analysis
to compute types for expressions as well and determine whether the code is
well formed. This stage is responsible for generating most of the compiler
warnings as well as parse errors. The output of this stage is an
"Abstract Syntax Tree" (AST).

Code Generation and Optimization

This stage translates an AST into low-level intermediate code (known as
"LLVM IR") and ultimately to machine code. This phase is
responsible for optimizing the generated code and handling target-specific
code generation. The output of this stage is typically called a
".s" file or "assembly" file.

Clang also supports the use of an integrated assembler, in which the code
generator produces object files directly. This avoids the overhead of
generating the ".s" file and of calling the target
assembler.

Assembler

This stage runs the target assembler to translate the output of the
compiler into a target object file. The output of this stage is typically
called a ".o" file or "object" file.

Linker

This stage runs the target linker to merge multiple object files into an
executable or dynamic library. The output of this stage is typically
called an "a.out", ".dylib" or ".so"
file.

Clang Static Analyzer

The Clang Static Analyzer is a tool that scans source code to try to find bugs
through code analysis. This tool uses many parts of Clang and is built into
the same driver. Please see < http://clang-analyzer.llvm.org> for
more details on how to use the static analyzer.

Select the Objective-C non-fragile ABI version to use by default. This
will only be used as the Objective-C ABI when the non-fragile ABI is
enabled (either via -fobjc-nonfragile-abi, or because it is the
platform default).

-fobjc-nonfragile-abi, -fno-objc-nonfragile-abi

Enable use of the Objective-C non-fragile ABI. On platforms for which this
is the default ABI, it can be disabled with
-fno-objc-nonfragile-abi.

Clang fully supports cross compilation as an inherent part of its design.
Depending on how your version of Clang is configured, it may have support for
a number of cross compilers, or may only support a native target.

-arch <architecture>

Specify the architecture to build for.

-mmacosx-version-min=<version>

When building for Mac OS X, specify the minimum version supported by your
application.

-miphoneos-version-min

When building for iPhone OS, specify the minimum version supported by your
application.

-march=<cpu>

Specify that Clang should generate code for a specific processor family
member and later. For example, if you specify -march=i486, the compiler is
allowed to generate instructions that are valid on i486 and later
processors, but which may not exist on earlier ones.

-O0 Means "no optimization": this level
compiles the fastest and generates the most debuggable code.

-O1 Somewhere between -O0 and -O2.

-O2 Moderate level of optimization which enables most optimizations.

-O3 Like -O2, except that it enables optimizations that take
longer to perform or that may generate larger code (in an attempt to make the
program run faster).

-Ofast Enables all the optimizations from -O3 along with other
aggressive optimizations that may violate strict compliance with language
standards.

-Os Like -O2 with extra optimizations to reduce code size.

-Oz Like -Os (and thus -O2), but reduces code size further.

-Og Like -O1. In future versions, this option might disable
different optimizations in order to improve debuggability.

-O Equivalent to -O2.

-O4 and higher

Currently equivalent to -O3

-g, -gline-tables-only, -gmodules

Control debug information output. Note that Clang debug information works
best at -O0. When more than one option starting with -g is
specified, the last one wins:

-g Generate debug information.

-gline-tables-only Generate only line table debug information. This
allows for symbolicated backtraces with inlining information, but does not
include any information about variables, their locations or types.

-gmodules Generate debug information that contains external references to
types defined in Clang modules or precompiled headers instead of emitting
redundant debug type information into every object file. This option
transparently switches the Clang module format to object file containers that
hold the Clang module together with the debug information. When compiling a
program that uses Clang modules or precompiled headers, this option produces
complete debug information with faster compile times and much smaller object
files.

This option should not be used when building static libraries for distribution
to other machines because the debug info will contain references to the module
cache on the machine the object files in the library were built on.

-fstandalone-debug -fno-standalone-debug

Clang supports a number of optimizations to reduce the size of debug
information in the binary. They work based on the assumption that the
debug type information can be spread out over multiple compilation units.
For instance, Clang will not emit type definitions for types that are not
needed by a module and could be replaced with a forward declaration.
Further, Clang will only emit type info for a dynamic C++ class in the
module that contains the vtable for the class.

The -fstandalone-debug option turns off these optimizations. This is
useful when working with 3rd-party libraries that don't come with debug
information. This is the default on Darwin. Note that Clang will never
emit type information for types that are not referenced at all by the
program.

-fexceptions

Enable generation of unwind information. This allows exceptions to be
thrown through Clang compiled stack frames. This is on by default in
x86-64.

-ftrapv

Generate code to catch integer overflow errors. Signed integer overflow is
undefined in C. With this flag, extra code is generated to detect this and
abort when it happens.

-fvisibility

This flag sets the default visibility level.

-fcommon, -fno-common

This flag specifies that variables without initializers get common
linkage. It can be disabled with -fno-common.

-ftls-model=<model>

Set the default thread-local storage (TLS) model to use for thread-local
variables. Valid values are: "global-dynamic",
"local-dynamic", "initial-exec" and
"local-exec". The default is "global-dynamic". The
default model can be overridden with the tls_model attribute. The compiler
will try to choose a more efficient model if possible.

-flto, -flto=full, -flto=thin, -emit-llvm

Generate output files in LLVM formats, suitable for link time
optimization. When used with -S this generates LLVM intermediate
language assembly files, otherwise this generates LLVM bitcode format
object files (which may be passed to the linker depending on the stage
selection options).

The default for -flto is "full", in which the LLVM bitcode
is suitable for monolithic Link Time Optimization (LTO), where the linker
merges all such modules into a single combined module for optimization.
With "thin", ThinLTO compilation is invoked instead.

These environment variables are checked, in order, for the location to
write temporary files used during the compilation process.

CPATH

If this environment variable is present, it is treated as a delimited list
of paths to be added to the default system include path list. The
delimiter is the platform dependent delimiter, as used in the PATH
environment variable.

To report bugs, please visit < http://llvm.org/bugs/>. Most bug
reports should include preprocessed source files (use the -E option)
and the full output of the compiler, along with information to reproduce.